Methane uptake and nitrous oxide emission in Japanese forest soils and their relationship to soil and vegetation types

Abstract

To determine the means and variations in CH₄ uptake and N₂O emission in the dominant soil and vegetation types to enable estimation of annual gases fluxes in the forest land of Japan, we measured monthly fluxes of both gases using a closed-chamber technique at 26 sites throughout Japan over 2 years. No clear seasonal changes in CH₄ uptake rates were observed at most sites. N₂O emission was mostly low throughout the year, but was higher in summer at most sites. The annual mean rates of CH₄ uptake and N₂O emission (all sites combined) were 66 (2.9–175) µg CH₄-C m^?2 h^?1 and 1.88 (0.17–12.5) µg N₂O-N m^?2 h^?1, respectively. Annual changes in these fluxes over the 2 years were small. Significant differences in CH₄ uptake were found among soil types (P < 0.05). The mean CH₄ uptake rates (µg CH₄-C m^?2 h^?1) were as follows: Black soil (95 ± 39, mean ± standard deviation SD]) > Brown forest soil (60 ± 27) ≥ other soils (20 ± 24). N₂O emission rates differed significantly among vegetation types (P < 0.05). The mean N₂O emission rates (µg N₂O-N m^?2 h^?1) were as follows: Japanese cedar (4.0 ± 2.3) ≥ Japanese cypress (2.6 ± 3.4) > hardwoods (0.8 ± 2.2) = other conifers (0.7 ± 1.4). The CH₄ uptake rates in Japanese temperate forests were relatively higher than those in Europe and the USA (11–43 µg CH₄-C m^?2 h^?1), and the N₂O emission rates in Japan were lower than those reported for temperate forests (0.23–252 µg N₂O-N m^?2 h^?1). Using land area data of vegetation cover and soil distribution, the amount of annual CH₄ uptake and N₂O emission in the Japanese forest land was estimated to be 124 Gg CH₄-C year^?1 with 39% uncertainty and 3.3 Gg N₂O-N year^?1 with 76% uncertainty, respectively.